In the same kind of way that a movie about Superman is going to be called Superman, it makes sense that A Gathering of Old Men is called A Gathering of Old Men because that's what the novel is focusing on. But you've always got to keep in mind that Gaines loves folding some more complicated stuff into all sorts of things that seem really simple at first, from character interactions to the way a specific scene is narrated. The title of his novel is no different.
First and foremost, just give the title a second—or third—think. What could be more harmless than a gathering of old men? Seems pretty innocent, doesn't it? It just so happens, though, that these guys aren't meeting up to watch a slideshow of Mathu's fabulous pleasure cruise to the Bahamas (just so we're clear, Mathu never goes to the Bahamas). Their toting shotguns and they're ready for a fight. There's more to the title than meets the eye, just like there's a whole lot more going on with the old men hanging out at Mathu's place. Sure, it's a safe bet that they're all there to support Mathu, but they've all got their own scores to settle and their own reasons for being fed up with bigots like Fix. That's not exactly the same thing as getting together to for a few rounds of shuffleboard.
And then there's the way the novel itself is written: as a series of voices coming together to make a single cohesive story. Sure, not all of those voices come from elderly folks—and not all of the voices belong to men (hearing from a few more women would have been cool, though)—but the chapters of the novel itself are a kind of gathering of different people and perspectives. It's Gaines's way of emphasizing and helping us appreciate the importance and strength of a sense of community among people of color even in the face of some seriously intense problems. And it just so happens that we think that's worth appreciating, too.